Universal Motor

The motors which can be used with a single phase AC source as well as a DC
source of supply and voltages are called as Universal Motor. It is also known
as Single Phase Series Motor

A universal motor is a commutation type motor. If the polarity of the line

terminals of a DC Series Motor is reversed, the motor will continue to run in the
same direction.

Thus, a Universal motor can work on both AC and DC. However, a series motor
which is mainly designed for DC operation if works on single phase AC supply
suffers from the following drawbacks.

 The efficiency becomes low because of hysteresis and eddy current losses.
 The power factor is low due to the large reactance of the field and the armature
windings.
 The sparking at the brushes is in excess.

In order to overcome the above following drawbacks, certain modifications are

made in a DC series motor so that it can work even on the AC current. They are
as follows:-

 The field core is made up of the material having a low hysteresis loss. It is
laminated to reduce the eddy current loss.
 The area of the field poles is increased to reduce the flux density. As a result, the
iron loss and the reactive voltage drop are reduced.
 To get the required torque the number of conductors in the armature is increased.
Permanent Magnet DC Motor or PMDC Motor
In a DC motor, an armature rotates inside a magnetic field. The basic working
principle of DC motor is based on the fact that whenever a current carrying
conductor is placed inside a magnetic field, there will be mechanical force
experienced by that conductor.
All kinds of DC motors work under this principle. Hence for constructing a DC
motor, it is essential to establish a magnetic field. The magnetic field is
established by using a magnet. You can use different types of magnets – it may
be an electromagnet or it can be a permanent magnet. A Permanent Magnet
DC motor (or PMDC motor) is a type of DC motor that uses a permanent
magnet to create the magnetic field required for the operation of a DC motor.
The cross-sectional view of the 2 pole PMDC motor is shown in the figure
below.

The Permanent Magnet DC motor generally operates on 6 V, 12 V or 24 Volts

DC supply obtained from the batteries or rectifiers. The interaction between the
axial current carrying rotor conductors and the magnetic flux produced by the
permanent magnet results in the generation of the torque.
The circuit diagram of the PMDC is shown below.
In conventional DC motor, the generated or back EMF is given by the equation
shown below.

The electromagnetic torque is given as

In Permanent Magnet DC motor, the value of flux ϕ is constant. Therefore, the

above equation (1) and (2) becomes

Considering the above circuit diagram the following equations are expressed.

Putting the value of E from the equation (3) in equation (5) we get

Where k1 = k ϕ and is known as speed-voltage constant or torque constant. Its

value depends upon the number of field poles and armature conductors.
Drive circuits:-
There are two drivers:-
(1) Unopolar drive
(2) Bipolar drive

Bipolar- Unipolar Driven Motor Working Analysis

In the bipolar working mode; the motor is driven by the direction change of
the current flowing through the phase windings in particular periods. In the
Bipolar working; the current flowing through the winding groups changes
direction during the commutation process depending on the rotor turn. Each
phase winding is controlled by two switching elements as shown in Figure 1

Fig:- 3 Phase Bipolar- Unipolar

Advantage of Brushless DC Motor

 BLDC motors have many advantages over brushed DC motors and

induction motors. A few of these are:
 Better speed versus torque characteristics
 Faster dynamic response
 High efficiency
 Long operating life
 Noiseless operation
 Higher speed ranges
  In addition, the ratio of torque delivered to the size of the motor is
higher, making it useful in applications where space and weight are
critical factors.

Stepper Motor and Control Circuits

Hysteresis Motor
A Hysteresis Motor is a synchronous motor with a uniform air gap and
without DC excitation. It operates both in single and three phase supply. The
Torque in a Hysteresis Motor is produced due to hysteresis and eddy current
induced in the rotor by the action of the rotating flux of the stator windings.
Operation of a Hysteresis Motor
The following illustration shows the basic functioning of a hysteresis motor.

When supply is given applied to the stator, a rotating magnetic field is

produced. This magnetic field magnetises the rotor ring and induces pole
within it. Due to the hysteresis loss in the rotor, the induced rotor flux lags
behind the rotating stator flux. The angle δ between the stator magnetic field
BS and the rotor magnetic field BR is responsible for the production of the
torque. The angle δ depends on the shape of the hysteresis loop and not on the
frequency.
Thus, the value of Coercive force and residual flux density of the magnetic
material should be large. The ideal material would have a rectangular
hysteresis loop as shown by loop 1 in the hysteresis loop figure. The stator
magnetic field produces Eddy currents in the rotor. As a result, they produce
their own magnetic field.
The eddy current loss is given by the equation shown below.

Where,
ke is a constant
f2 is the eddy current frequency
B is the flux density
As we know,

Where s is the slip and f1 are the frequency of the stator.

Therefore,

The torque is given by the equation shown below.

Now, the torque due to hysteresis loss is given by the equation shown below.

The Torque due to hysteresis is given as

From the equation (1) it is clear that the torque is proportional to the slip.
Therefore, as the speed of the rotor increases the value of Ʈe decreases. As the
speed of the motor reaches synchronous speed, the slip becomes zero and
torque also become zero.
Switched Reluctance Motor (SRM)
The switched reluctance motor (SRM) is a type of motor doubly salient with
phase coils mounted around diametrically opposite stator poles. There are no
windings or permanent magnets on the rotor. The rotor is basically a piece of
(laminated) steel and its shape forms salient poles. The stator has concentrated
coils.
Depending on their applications, SRMs are produced within a wide range of
structures. Presents a classification of these motors based on their movement
patterns, flux path, and type of excitation, each being examined in the following
sections.
Electrostatic motor
An electrostatic motor is based on the attraction and repulsion of electric
charge. Usually, electrostatic motors are the dual of conventional coil-based
motors. They typically require a high voltage power supply, although very
small motors employ lower voltages. Conventional electric motors instead
employ magnetic attraction and repulsion, and require high current at low
voltages. In the 1740s and 1750s, the first electrostatic motors were developed
by Andrew Gordon and by Benjamin Franklin. Today the electrostatic motor
finds frequent use in micro-mechanical (MEMS) systems where their drive
voltages are below 100 volts, and where moving, charged plates are far easier
to fabricate than coils and iron cores.
An electrostatic motor or capacitor motor is a type of electric motor based on
the attraction and repulsion of electric charge.

Building and operation instructions:

Structure:
I recommend using wood as main construction material, as it is a good enough
insulator, cheap and easy to work with. If you use woodscrews, make sure they
don’t come closer than about two inches to the HV-carrying parts. The structure
mostly consists of a stable frame on which the upper and lower bearings and the
stator panels can be mounted. I have used nylon screws to mount the stator panels.
I recommend mounting the HV circuitry on the outside of the frame and covering
it with plexiglass. This will result in a compact package with a nice view onto the
HV parts. The plexiglass cover will allow you to explain the circuitry to spectators
while keeping the HV mostly contained to where it belongs.
Repulsion Motor
Definition: Arepulsion motor is a single-phase electric motor that operates by
providing input AC (alternating current). The main application of repulsion
motor is electric trains. It starts as a repulsion motor and runs as an induction
motor, where the starting torque should be high for repulsion motor and very
good running characteristics for induction motor.

Classification of Repulsion Motor

There are three types of repulsion motor they are,
Compensated Type
It consists of an additional winding namely compensating winding and an
additional pair of brushes are placed between the (short-circuited) brushes.
Both compensating winding and a pair of brushes are connected in series for
improving the power and speed factors. A compensated type motor is used
where there is required for high power at the same speed.
Repulsion Start Induction Type
It starts with the repulsion of coils and runs with the induction principle, where
speed is maintained constant. It has a single stator and rotor similar to DC
armature and a commutator where a centrifuge mechanism short-circuits the
commutator bars and has higher torque (6 times) than the current in the load. The
operation of repulsion can be understood from the graph that is, when the
frequency of synchronous speed increases, the percentage of full torque load starts
decreasing, where at a point the magnet poles experience a repulsive force and
switches into induction mode. Here we can observe the load that is inversely
proportional to speed.
Repulsion Type
It works on the principle of repulsion and induction, which consists of a stator
winding, 2 rotors winding (where one is squirrel cage and other DC winding).
These windings are shorted to commutator and two brushes. It operates in a
condition where the load can be adjustable and whose starting torque is 2.5-3.

Synchros and Control Transformers

A synchro control transformer is used in conjunction with a synchro transmitter
to act as error sensor of mechanical components. Basically, the construction of
a synchro control transformer is very similar to that of the synchro transmitter,
except that the rotor is cylindrically shaped so that the air gap flux is uniformly
distributed around the rotor.

Application:
For the application of detection of the error, the synchros transformer should
be connected to the synchros detector. The stator leads of the synchro
transmitter are connected to the stator leads of a synchro control transformer.

The rotor voltage of the control transformer is approximately proportional to

the difference between the positions of the rotors of the transmitter and the
control transformer. For small discrepancies of the controlled and reference
shaft positions, the transfer function of the synchro error detector can be written
as:
E = error voltage
θr = reference shaft position in degrees
θc = controlled shaft position in degrees
θe = θr – θc = error in angular positions between the reference shaft and the
controlled shaft in degrees
Ks = sensitivity of the error detector in volts per degree.